Depicted in FIG. 1 is an ink jet colour printer on which the main parts are labelled as follows: a fixed structure 41, a scanning carriage 42, an encoder 44 and, by way of example, printheads 40 which may be either monochromatic or colour, and variable in number.
The printer may be a stand-alone product, or be part of a photocopier, of a “plotter”, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium 46, normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar.
Also shown in FIG. 1 are the axes of reference:
x axis: horizontal, i.e. parallel to the scanning direction of the carriage 42; y axis: vertical, i.e. parallel to the direction of motion of the medium 46 during the line feed function; z axis: perpendicular to the x and y axes: i.e. substantially parallel to the direction of emission of the droplets of ink.
The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on some only of the features of the heads and the manufacturing process, of relevance for the purposes of understanding this invention.
The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a definition of more than 600 dpi (dpi=“dots per inch”), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies.
Requirements such as these are especially important in colour printhead manufacture and make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, narrow assembly tolerances. It is important in particular to ensure that the volume and speed of the droplets subsequently emitted are as constant as possible, and that no “satellite” droplets are formed as these, with a trajectory generally different from the main droplets, are distributed randomly near the edges of the graphic symbols, reducing their sharpness.
FIG. 2 shows an enlarged axonometric view of an actuating assembly 111 of an ink jet printhead according to the known art, made of a die 100 of semiconductor material (usually Silicon), on the upper face of which resistors 27 have been made for emission of the droplets of ink, driving circuits 62 for driving the resistors 27, soldering pads 77 for connecting the head to an electronic controller not shown in the figure, and which bears a pass-through slot 102 through which the ink flows from a reservoir not shown in the figure. Around the upper edge of the slot 102 a basin 76 has been made, the characteristics and functions of which are as described in detail in Italian patent application TO 98A 000562. Affixed to the upper face of the die is a layer 105 of photopolymer having, usually though not exclusively, a thickness less than or equal to 25 μm in which, by means of known photolithographic techniques, a plurality of ducts 53 and a plurality of chambers 57 positioned locally to the resistors 27 having been made. Stuck on the photopolymer 105 is a nozzle plate 106, generally made of a plate of gold-plated nickel or kapton, of thickness less than or equal to 50 μm, bearing a plurality of nozzles 56, each nozzle 56 being in correspondence with a chamber 57. In the current technology, the nozzles 56 have a diameter D of between 10 and 60 μm, while their centres are usually spaced apart by a pitch A of 1/300th or 1/600th of an inch (84.6 μm or 42.3 μm). Generally, though not always, the nozzles 56 are arranged in two rows parallel to the y axis, staggered one from the other by a distance B=A/2, in order to double the resolution of the image in the direction parallel to the y axis; the resolution thus becomes 1/600th or 1/1200th of an inch (42.3 μm or 21.2 μm). The x, y and z axes, already defined in FIG. 1, are also shown in FIG. 2.
FIG. 3 is an axonometric enlargement of two chambers 57, adjacent and communicating with the slot 102 through the basin 76 and the ducts 53 made in the layer of photopolymer 105. Normally the ducts 53 have a length l and a rectangular cross-section having a depth a and a width b. The chambers 57 have a depth d, substantially equal to the depth a of the ducts 53.
A section of an ejector 55 can be seen in FIG. 4, where the following are shown, in addition to the items already mentioned: a reservoir 103 containing ink 142, a droplet 51 of ink, a vapour bubble 65, a meniscus 54 in correspondence with the surface of separation between the ink and the air, an external edge 66 and arrows 52 which indicate the prevalent direction of motion of the ink.
To describe the operation of an ejector for a thermal type ink jet printhead, an electrical analogy is used, for which the following equivalences are established:
V=electrical voltage in volt equivalent to: pressure in N/M2;
I=current in A equivalent to: flow rate in m3/s;
R=resistance in ohm equivalent to: hydraulic resistance in
N/m2/m3/s=N s/m5;
L=Inductance in henry equivalent to the ratio between the mass of the column of liquid that fills the duct and the square of the section of the duct; this ratio is called “hydraulic inertance”, and is measured in kg/m4;
C=capacitance in farad equivalent to: hydraulic compliance
in m3/N/m2=m5/N.
In the equivalent diagram of FIG. 5 the bubble is represented as a variable capacitance Cb. There is a front leg 70, equivalent to the whole formed by the chamber 57, the nozzle 56, the meniscus 54 and the droplet 51, and a rear leg 71, which represents the section of the hydraulic circuit between the chamber 57 and the reservoir 103.
The front leg 70 comprises a fixed impedance Lf, Rf corresponding substantially to the chamber 57, a variable impedance Lu, Ru corresponding substantially to the nozzle 56, and a deviator T which, during the step in which the droplet 51 is formed, inserts a variable resistance Rg substantially corresponding to the droplet, whereas, during the steps of withdrawal of the meniscus 54, of filling of the nozzle, of subsequent oscillation and damping of the meniscus, inserts a capacitance Cm substantially corresponding to the meniscus itself.
Ejection of the ink takes places in accordance with the following steps:
a) The electronic control circuit 62 supplies energy to the resistor 27, so as to produce local boiling of the ink with formation of the bubble 65 of steam in expansion. During this step, in the equivalent electric circuit of FIG. 5 the variable resistance Rg is inserted. The bubble 65 generates two opposing flows: Ip (to the reservoir 103) and Ia (to the nozzle 56).
b) The electronic circuit 62 terminates the delivery of energy to the resistor 27, the vapour condenses, the bubble 65 collapses, the droplet 51 detaches itself, the meniscus 54 withdraws emptying the nozzle 56. The two opposing flows Ip and Ia remain. In this step, in the equivalent circuit of FIG. 5 the capacitance Cm corresponding to the meniscus 54 is inserted.
c) The bubble 65 has disappeared, the meniscus 54 demonstrates its capillarity and goes back towards the outer edge 66 of the nozzle 56 sucking new ink 142 into the nozzle 56. Its return completed, the meniscus 54 remains attached to the outer edge 66 by oscillating and behaving like a vibrating membrane. In the equivalent electric circuit of FIG. 5 the capacitance Cm is still inserted. During this step the equivalent circuit of the ejector 55 is simplified as sketched in FIG. 6, where Cm represents the capacitance of the meniscus, while R and L represent respectively the sum of all the resistances and of all the inductances present between the meniscus 54 and the reservoir 103. In addition, the flows Ip and Ia converge into a single flow i.
To obtain an optimal operation of the ejector 55, it is necessary for the meniscus 54, at the end of the step c), to reach the idle state rapidly and without oscillating. In this way the ink 142 does not wet the outer surface of the nozzle plate 106, thereby avoiding alterations of speed and volume of the following droplets.
For a given nozzle 56 the parameters Lu, Ru and Cm, belonging to the front hydraulic part 70 of the ejector 55, are set and therefore, to obtain the values of R and L according to the criteria set down below, it is possible to act only on the design of the rear hydraulic part 71.
The expression in function of the time i, which represents the flow, is given by the known relation:
                    i        =                                            V              m                        L                    *          t          *                      ⅇ                                          -                t                                            2                ⁢                τ                                                                        (        1        )            where Vm represents the pressure generated by the meniscus 54, which is negative during the filling step, and τ is the time constant, measured in seconds, of the RLC circuit of FIG. 6, equal to the ratio L/R.
For maximum speed in filling of the nozzle 56, the flow i must be rendered maximal, and for this to happen L and τ must be rendered minimal.
Also, for the meniscus 54 to reach the idle state rapidly without oscillating, the equivalent circuit of FIG. 6 must be “critical damping” type, and must for this purpose satisfy the known relation:
                    R        =                  2          *                                    L                              C                m                                                                        (        2        )            
For a duct 53 of length l, the section of which has sides a and b with a >>b, the following known relations apply:
                    R        ≅                              12            *            ρ            *            v            *            l                                              b              3                        *            a                                              (        3        )                                L        ≅                              ρ            *            l                                b            *            a                                              (        4        )                                τ        =                              L            R                    =                                    b              2                                      12              *              v                                                          (        5        )            where ρ is the density of the ink in kg/m3, v is the viscosity of the ink in m2/s, and all lengths are measured in meters.
The time constant τ is a function of the width b, while it is independent of both the depth a and the length l.
It is possible to determine a value of b which gives values R and L such as to produce the critical damping, according to the expression (2). However the same value of b, substituted in (5), provides a value of τ which limits the flow i, according to the relation (1), and accordingly limits the emission frequency of the droplets. Moreover, it is not possible to modify either depth a or length l at will, as these parameters are subject to other technological and functional constraints, not described as they are not essential for the understanding of this invention.
To increase the emission frequency of the droplets, it is necessary to make the time constant τ much shorter than that obtained in the known art, while at the same time satisfying the critical damping condition: this problem is solved in this invention by making a plurality of N ducts in parallel, as will be seen in detail in the description of the preferred embodiment.
Some further drawbacks with the chambers 57 according to the known art are now mentioned, which have three continuous lateral walls and a fourth wall interrupted by the duct 53 of non-negligible width. In this situation the bubble 65 collapses prevalently in the direction of the resistor 27 underneath, which is thus subjected to greater wear on account of the known phenomenon of cavitation. In addition, the collapse of the bubble is dissymmetrical as it is attracted to the wall opposite the duct 53: this cause a dissymmetry in the motion of the meniscus 54, with a resulting deviation of the terminal part of the droplet 51 and the formation of satellite droplets having a different direction from the droplet 51.
In this invention the duct 53 is substituted by N ducts placed in parallel and communicating with the chamber through the lower or upper wall, and consequently the four lateral walls of the chamber are continuous and symmetrical.
In U.S. Pat. No. 5,666,143 a solution is described in which the ink is brought to the chamber along multiple ducts, but these do not suffice to solve the problems reported.